[fixup] map /sys/kernel/debug as a volume for the containers

.travis.yml

@@ -27,4 +27,4 @@ matrix:
 
 script:
   - docker build -t bpftrace-builder-$BASE -f docker/Dockerfile.$BASE docker/
-  - docker run --rm -it -u $(id -u):$(id -g) -v $(pwd):$(pwd) -e STATIC_LINKING=$STATIC_LINKING -e TEST_ARGS=$TEST_ARGS bpftrace-builder-$BASE $(pwd)/build-$TYPE-$BASE $TYPE
+  - docker run --rm -it -u $(id -u):$(id -g) -v $(pwd):$(pwd) -v /sys/kernel/debug:/sys/kernel/debug -e STATIC_LINKING=$STATIC_LINKING -e TEST_ARGS=$TEST_ARGS bpftrace-builder-$BASE $(pwd)/build-$TYPE-$BASE $TYPE